1. Field of the Invention
The present invention relates to an ultrasonic inspection method, an ultrasonic test method and an ultrasonic inspection apparatus. The invention more particularly relates to an ultrasonic inspection method, an ultrasonic test method and an ultrasonic inspection apparatus which are suitable to evaluate the height of a minute defect.
2. Description of the Related Art
Many metal devices and structures exist in electric power plants and chemical plants. Cracks (defects) such as fatigue cracks and stress corrosion cracking may occur on the surfaces of metal devices and structure and develop inside the plants. In order to ensure the soundness of the metal devices and structures, it is necessary to execute a non-destructive inspection; a visual inspection, an inspection with radiation transmission, an ultrasonic inspection and the like are executed for that purpose. In addition, there is an increasing demand to not only determine the presence or absence of a defect but also accurately evaluate the height of a defect developing in the metal devices and structures.
For a main pipe and the like of the power generating plant, for example, part at which a crack is likely to occur and direction in which the crack will develop can be estimated to some extent by stress analysis or the like though they vary depending on a substance, an environment, stress and the like. In contrast, when the ultrasonic inspection is used, it is difficult to visualize the details of the defect. If a wave reflected from an edge (aperture of a defect) at which the defect has occurred is detected by the ultrasonic inspection, however, then the degree of development of the defect in a sample can be evaluated by carefully detecting a wave diffracted from the distal end of the defect. Thus, emphasis is placed on an ultrasonic inspection whose inspection system is relatively simple is used and that enables information on the inside of the structure almost to be acquired.
Conventional ultrasonic inspection methods for sizing include a single probe method (conventional method) and a double-probe method. The single probe method is an inspection method for transmitting and receiving an ultrasonic wave by the same probe using a probe (hereinafter referred to as single probe) provided with a single element or a probe (hereinafter referred to as array probe) provided with multiple elements. The double-probe method is an inspection method for transmitting and receiving an ultrasonic wave by combined two probes, which are a probe for transmitting an ultrasonic wave and a probe for receiving an ultrasonic wave (refer to, for example, Non-Patent Document 1, “Method for measuring the height of a defect using tip echo method” issued by The Japanese Society for Non-Destructive Inspection, on Jun. 1, 1997). Two units of a single probe or two units of an array probe may be combined as the probes to be used for the double-probe method. A single probe and an array probe may be combined. The conventional single probe method to be used for sizing is an angle beam method. The angle beam method is to cause an ultrasonic wave transmitted by a single angle probe or an array probe to be incident on a surface (to be inspected) in an oblique direction, cause the same probe to receive a wave (corner echo) reflected from an aperture of the defect and a wave (tip echo) diffracted from the tip of the defect, locate the position of the probe receiving the waves with the maximum intensities, and evaluate the height of the defect from the path lengths of the diffracted wave and the reflected wave.
Proposed conventional double-probe methods include a TOFD method, an SPOD method, a delta method, and a CAFS method.
Sizing based on the TOFD method is as follows. A transmitting probe and a receiving probe are arranged so that the defect lies between the transmitting probe and the receiving probe, an ultrasonic wave is transmitted and received therebetween, and a diffracted wave is received, thereby evaluating the height of a defect (refer to, for example, Non-Patent Document 2, “Method for measuring the height of a defect using TOFD method” issued by the Japanese Society for Non-Destructive Inspection, on Dec. 1, 2001).
Sizing based on the SPOD method is as follows. An angle probe is used as a transmitting probe so as to transmit an ultrasonic wave at an oblique angle while a vertical probe is used as a receiving probe so as to receive the ultrasonic wave in a vertical direction. The ultrasonic wave transmitted from the angle probe is incident on a defect in an oblique direction so as to cause a wave to be diffracted from the tip of the defect. The vertical probe located above the defect receives a diffracted wave directly propagating above the defect and a diffracted wave reflected (skipped) on a back surface once and propagating above the defect after the reflection, and thus the height of the defect from the back surface of the tip of the defect is evaluated based time periods for the propagations of the diffracted waves (refer to, for example, JP-2007-315820-A).
Sizing based on the delta method is one of mode conversion methods and is as follows. An angle probe is used as a transmitting probe so as to transmit an ultrasonic wave at an oblique angle while a vertical probe is used as a receiving probe so as to receive the ultrasonic wave in a vertical direction in the same manner as the SPOD method. The angle probe transmits a 60-degree transverse wave. The vertical probe receives a 0-degree longitudinal diffracted wave subjected to mode conversion at the tip of the defect after being skipped at a back surface, and thus the height of the defect is evaluated (refer to, for example, Non-Patent Document 3, “Ultrasonic test method, P. 249, (issued on Jul. 30, 1974 by The Nikkan Kogyo)” and Non-Patent Document 4, “Introduction to phased array ultrasonic technology applications, P. 84 (issued in August, 2004 by R/D Tech Inc.)”.